Number of Cells OK? - RC Groups
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Oct 12, 2017, 08:22 PM
Here Comes the Sun
Help!

Number of Cells OK?


I converted a 40 size Hangar 9 Stik to electric. Power is a Rimfire 46 with a 60 amp ESC. Prop is within the Rimfires recommended range. Towers website says to use 5-6 cells. However Ive been using 4 cells. The Stik flies beautifully. Batteries are slightly warm and charge readily. Is there a danger that I dont see in using the 4 cell vs the 5 or 6 cell?
Last edited by dhable; Oct 12, 2017 at 08:29 PM.
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Oct 12, 2017, 10:15 PM
Registered User
scirocco's Avatar
No risk here. If the current through the motor and input power is less than the max specified for motor and esc, you can run on any voltage that turns the prop you choose at the rpm you want.

Your application just demonstrates the flexibility of an electric system to achieve a wide range of solutions and also the limited scenarios that vendors consider when they publish their motor data.
Oct 13, 2017, 06:29 AM
A man with too many toys
The reason they don't recommend 4 cells for the 46 is because they have the 32. The Rimfire 32 is exactly the same Kv and bolt pattern but less weight. You don't need as much mass if you are using less watts. Also the 32 has a lower Io so it will be more efficient.

Rimfire 46 will work you will just carry more weight and have less flight time.

What prop are you using?

.
Oct 13, 2017, 08:24 AM
Here Comes the Sun
10x5

Im a glow guy and to me on the surface of it, a rimfire 32 sounds like it has a lot less power than a 46. Am I tbinking correctly? Are you recommending I switch the 46 out for a 32?
Oct 13, 2017, 10:33 AM
An itch?. Scratch build.
eflightray's Avatar
Dropping the number of cell than what the manufacturer 'recommends', is definitely not a problem, (usually it's their max number of cells to stop the 'magic smoke' escaping).

Sometime a heavier motor is handy as it mean it's less stressed, and will help getting the CG right on a conversion, (you don't have to add lead).

On my Dauntless, I'm using a motor normally rated as 5s to 6s, and I'm running it on 3s. But in now turns a 17" x 8" prop and still at quite low amps. Loads of thrust, (speed isn't important to me, but there is enough).

One recommendation, get a wattmeter, you will probably find you can run a much bigger diameter prop and still be well within the max amps.

Ray.
Oct 13, 2017, 11:50 AM
A man with too many toys
Quote:
Originally Posted by dhable
10x5

Im a glow guy and to me on the surface of it, a rimfire 32 sounds like it has a lot less power than a 46. Am I tbinking correctly? Are you recommending I switch the 46 out for a 32?
No, not realy.

They both have the same Kv so they both have about the same power using the same prop. To make an analytical calculation comparison you need the Rm and Io for the motors.

Advantage of the larger motor: It will take more power (max watts). You are way-way below the limit on your setup.

Disadvantage of larger motor: Higher Io for the larger motor. Io is the no load current, it's basically lost energy. It's 2.6 Amps for the 32 and 4.6 amps for the 46. The 46 is heavier (268g vs 198g) so you same 70g of dead weight.

I am not recommending that you spend more money and get the 32. Just saying that for future purchases don't get a larger motor than you need.

.
Oct 13, 2017, 05:44 PM
Registered User
scottc39's Avatar
I have ran the rimfire .46 with a 12x8 and is well within the motor specs on 4 cells, the 10x5 is really cheating your sport planes power and fun
Oct 13, 2017, 06:05 PM
Registered User
scirocco's Avatar
Quote:
Originally Posted by dhable
10x5

Im a glow guy and to me on the surface of it, a rimfire 32 sounds like it has a lot less power than a 46. Am I tbinking correctly? Are you recommending I switch the 46 out for a 32?
Glow equivalent labels never tell the whole story, because the actual prop and battery that the vendor is using to draw that equivalence is rarely specified. If you use a different prop and battery combination than the vendor assumed you can get very different outcomes counter-intuitive to the label numbers.

And the labels vary greatly across manufacturers. A Rimfire .32 and a Eflite Power 32 are similar in all key respects, but a Rimfire ,46 and Eflite Power 46 while similar physically differ greatly in Kv (800 rpm/V vs 670 rpm/V) - Great Planes in their larger motors typically assumes you are going to run smaller props at higher rpm to get the assumed outcome. Probably a valid direct glow equivalent approach, but potentially less efficient than exploiting bigger props turning slower as long as they will fit.

Much more important than the label number is the motor Kv and weight. Kv drives the relationship between battery voltage, prop used and power consumed, while weight is a good indicator of maximum power handling capability.

In this case, with Kv the same and the difference between the 32 and 46 being just size and weight, for the same battery and props up to the max reasonable power handling limit of the 32 (about 800W), they are going to perform about the same. The difference comes in when you when you need to run a prop/voltage combination that pushes above the 32 limit, but is still within the safe limits of the 46.

So at modest loadings, they might as well have the same label. The Ecalc motor simulator predicts prop rpm the same within 0.5% and input power the same within 5%. A few more rpm and thus thrust from the .46 at the expense of 20W extra. But push up to around 65A on 4S (about 12x10 will do that) and the 46 becomes a couple % more efficient.

Although the 32 has lower no load current as noted by RC Man and is indeed fractionally more efficient at the ~350W on 4S a 10x5 draws, the difference is barely significant because Io is only part of the story on motor losses. Resistance is the other part and the 46 has lower resistance, most likely because being a longer motor, fewer turns of wire were required to get the same Kv as the shorter 32.

If your Stik is balancing nicely with the 46 and you're happy with the performance, no reason to change at all, and if you want a ballistic Stik, you've got the headroom in the motor for easily double the power you're presently using

ps a good example of the deficiency of glow equivalent labeling is comparing the Eflite Power 25A (870 rpm/V) and Power 32 (770 rpm/V), which are often used a standard power and sporty power options for many Horizon models. But they weigh the same and for the same battery and prop up to 4S 11x8, which is about the spec max for the Power 25, the Power 25 is a significantly more powerful setup than the Power 32.


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